Image Quiz: Ophthalmology—A squinting cattle dog

This is an outdoor 5-year-old castrated male Australian cattle dog that just started showing signs of severe squinting in his left eye. The dog has had similar problems before, but the caregiver is not the owner and does not remember the details—just that the dog gets better when it is taken to the veterinarian. The regular veterinarian is unavailable, so the dog is brought to you. What is the most likely diagnosis?